Microstructural Characterization of Aged Welded Joints of a UNS S32750 Superduplex Stainless Steel

Superduplex stainless steels present an excellent combination of high mechanical strength and corrosion resistance, obtained through the microstructural balance of ferrite (δ) and austenite (γ). Such a balance of properties, which allows important applications of these alloys, can be drastically affected in welding operations, due to precipitation of secondary phases that lead to unbalance between the fractions of the previous constituent phases. These secondary phases, such as sigma (σ) and chi (χ), can spoil important properties for the application of these steels in industry, mainly in the petroleum and gas production pipes, such as corrosion resistance and fracture toughness. The use of field emission scanning electron microscopy and WDS (dispersive x-ray wavelength) techniques allowed the evaluation of the microstructure at different temperatures of aging, mainly another possibility of identification of these harmful phases. The results showed the strong effect of the temperature increase on the microstructural changes in UNS S32750 superduplex stainless steel welded joint, due to precipitations of the sigma (σ), chi (χ) and secondary austenite (γ₂) phases, thus contributing to a better understanding of the aging phenomena at these temperatures. The use of WDS technique allowed obtaining more quantitative data, thus contributing to the discussion on the chemical composition of the phases.